Reverse osmosis (RO) and nanofiltration (NF) membranes have been used extensively in water treatment, wastewater reclamation and separation of organic pollutants from aqueous streams in the last few decades. The reverse osmosis (RO) membrane of choice worldwide is the polyamide (PA) thin film composite membrane.
Most commercial NF and RO membranes have a composite structure consisting of three layers: a thin selective polyamide (PA) layer that is a few hundred nanometers in thickness, a microporous polysulfone support layer, and a non-woven fabric layer for mechanical strength, as shown in FIG. 1(a). FIG. 1(b) depicts a transmission electron micrograph (TEM) of a virgin membrane where the dense PA layer and the porous polysulfone layer are clearly visible. This composite design renders possible the separate optimization of performance and mechanical stability. The thin selective layer is reportedly formed via interfacial polymerization of amine monomers (usually 1,3-benzenediamine or m-phenylene diamine (MPD)) in an aqueous solution reacting with trimesoyl chloride (1,3,5-benzentricarbonyl chloride (TMC)) in an organic solvent, according to the chemical reaction depicted in FIG. 2. This technique is based on a polycondensation reaction between two monomers, the polyfunctional amine and the acid chloride, which are dissolved in immiscible solvents. The aqueous amine solution initially impregnates the support. An ultrathin film (skin), with thickness under half a micron, is quickly formed at the interface and remains attached to the support. The reaction is believed to take place at the organic side of the interface due to the negligible solubility of acid chlorides in water and the good solubility of amines in organic solvents. In this reaction scheme, both the amine and acid chloride monomers are aromatic, and the three —COCl groups in the trimesoyl chloride allow the resulting fully aromatic PA membrane to be highly crosslinked, which is important for the high salt rejection required by RO membranes. The polyamide layer formed in this way is relatively rough, due to the ridge-and-valley structures [see FIG. 1(b)] with a roughness in the range of 100 nm.
Although the membrane performance is generally characterized as very good to excellent, significant membrane deficiencies may exist that can contribute to reduced membrane life and higher operating costs. Some indicators of membrane performance decline include an increase in the salt passage (i.e., reduction in salt rejection), an increase of differential pressure, and a reduction in permeate (product) flow. The disadvantages reported also include lack of chemical stability to oxidants such as chlorine, high fouling rates due to surface roughness, and high bacteria attachment on the membrane surface leading to bio fouling. Moreover, additional deficiencies can also include pinholes, defects, or similar ‘weak’ spots on the skin, due to non-uniform coverage during the manufacturing process, which tend to be aggravated with operating time and after repeated contact with cleaning chemicals. These drawbacks lead to membrane performance deterioration and increased frequency of replacement.
The physicochemical properties of NF/RO membrane active layers such as surface roughness, thickness, chemical functionality, charge and degree of cross-linking affect interactions with water and solutes, thus impacting on membrane performance. PA membranes have a relatively high degree of surface roughness, usually determined by characterization through Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) (FIGS. 3 and 4). These figures show that the PA membrane (FIG. 3) has a ridge-valley type of structure, while for the XLE membrane (FIG. 4), which has been characterized as a fully aromatic membrane, the mean roughness has been estimated to be 73 nm, whereas the reported RMS roughness is 142.8 nm. For other membranes, the RMS roughness was estimated to be 117 nm. Recent studies have shown that membrane surface morphology and structure influence performance characteristics of membranes. It has been suggested that an approximately linear relationship exists between membrane surface roughness and permeate flux for crosslinked aromatic polyamide reverse osmosis (RO) membranes, where permeability increases with increasing surface roughness. The linear relationship is attributed to surface unevenness of the RO membrane skin layer, which results in enlargement of the effective membrane area.
The physicochemical properties of a very thin PA active layer essentially determines the performance (flux and rejection) of thin film composite (TFC) membranes. For example, membrane zeta potential has been used to correlate the transport of some trace organic solutes through RO and NF membranes. In addition, the flux performance and fouling behavior of a membrane may also be affected by its zeta potential. Other properties, such as the chemical composition and morphology of the polyamide layer, are also important to RO membrane performance. Consequently, very good understanding of the physicochemical properties of the polyamide layer becomes critical for developing methods to control membrane fouling and trace organics rejection. Unfortunately, the processes and exact chemistries for producing commercial RO membranes are generally proprietary, which greatly limits membrane users' understanding of the physical and chemical properties of these membranes.
It has been recognized long ago that weak spots or defects (pin holes) are not uncommon on the membrane surface, which naturally tend to degrade membrane element rejection characteristics. Recent research has shown that the PA active layer thickness is characterized by great spatial nonuniformity, exhibiting variations of the porosity and charge across the layer. The active layer mean thickness for well-known membranes in the market, varies in the range 100 nm-300 nm with a minimum thickness that may be as small as 50 nm. Furthermore, it is well-known that due to periodic chemical cleaning, membrane aging, and occasional chlorine attack, there is gradual membrane degradation, very likely due to weak spots or defects of the active layer. Membranes are subject to fouling, scaling, and aging, which all affect the membrane performance. Membrane aging and degradation are considered a manifestation of physico-chemical changes occurring in the active layer (i.e., agglomeration and clustering of polymeric species) that lead to reduced salt rejection.
Attempts have been made to apply a post-treatment, in the manufacturing process of hollow fiber membrane elements (“permeators”) used for brackish and seawater desalination, respectively. In this treatment, PT-A (polyvinyl methyl ether) and PT-B (tannic acid) were used. It was reported that PT-A increased salt rejection by reducing salt flow through membrane or fiber imperfections; PT-B was considered to be adsorbed on the membrane surface, thus enhancing salt rejection. The attempts at post-treatment of membrane systems, however, were only performed on hollow fiber membrane elements, not other types of membranes, such as spiral wound elements.
A need exists for a process that can be used post-manufacturing to help rejuvenate used or degraded membranes. It would be helpful if such processes could be used on various types of membranes, such as spiral wound membranes.